The prior art has disclosed a number of variants of a device of this type, in which an electric motor is used as drive for carrying out the mechanical action. However, the said treatment media have the drawback of dissolving many polymers therein. This can give rise to problems with regard to sticking, softening and expansion or even bursting of the polymer if it comes into contact with the treatment medium. This problem becomes more serious the higher the density of the treatment medium, such as for example in liquid or supercritical phases of the treatment medium under ever higher pressures. This may give rise to considerable problems in the event of an electric motor coming into contact with treatment media of this type, for example short-circuiting as a result of the electrical insulation of the motor windings being affected, leaks caused by damage to any sealing rings, and jamming caused by fretting of plastic bearing components. Furthermore, treatment media of this nature are able to dissolve may oils and greases. This leads to removal of grease from anything which comes into contact with the treatment medium. This problem is likewise exacerbated as the density of the treatment medium rises. In the case of the electric motor, this can give rise to problems with lubricated surfaces in the bearing of the motor shaft, as well as with any bearings used in the mechanical actuator.
To avoid the above complications, the electric motor can be positioned outside the sphere of influence of the treatment medium, i.e. outside the pressure chamber. The mechanical actuator provided inside the pressure chamber is then driven by a drive shaft which passes through a wall of the pressure chamber. However, this presents specific problems with the need to seal the drive shaft. Sealing problems of this nature are exacerbated at higher pressures of the treatment medium, resulting in an expensive structure.
An alternative is to make use of a magnetic coupling between the electric motor and the mechanical actuator, as shown for example in U.S. Pat. No. 5,267,455. There is no need for seals in this case. However, at high pressures, the wall thickness of the pressure chamber increases and consequently the magnetic coupling is more difficult to realize, certainly if high powers need to be transmitted. This leads to very expensive structures.
WO 00/63483 also discloses a device showing a washing machine drum which is driven by an electric motor. In this case, the washing machine drum and the electric motor are both located in a treatment space which is partially filled with highly pressurized liquid CO2 at room temperature. The document describes the electric motor having to be disposed as high as possible within the top part of the treatment space, so that the electric motor substantially, i.e. barring a few splashes of liquid CO2, only comes into contact with gaseous CO2.
One drawback in this case is that this known device is susceptible to faults and problems may still arise as a result of standard polymer components of the electric motor being adversely affected. Furthermore, the working range of the permitted pressure and temperature are limited, and consequently in some cases so is the effectiveness of the washing treatment which is intended in this case.